This invention relates generally to a gas-fired burner for use in industrial furnaces, processes or the like.
With increasing energy costs, more sophisticated processes and stricter emission codes, greater demands are being made on burner performance. Burner designs must be more sophisticated in order to meet industrial process turndown requirements which are sometimes in the range of 100:1. Not only must the burner be capable of operating over this wide turndown range but, while doing so, it must maintain short flame lengths, low levels of emission of nitrogen oxides and carbon monoxide, and a relatively low noise level. In addition, the design must be such as to make the cost of the unit competitive with other equipment on the market.
In its simplest form, a burner consists of some type of combustion air manifold, a gas manifold, and some type of flame retention device. The flame retention device is a major factor in determining the operational characteristics of the burner. The earliest forms of burners used a hot refractory burner block in conjunction with the scrubbing action of the flame against the block for flame retention. Since that time, the trend has been toward having some type of flame retention nozzle that does not depend on hot refractory or scrubbing action.
One of the simplest forms of a burner with a flame retention nozzle employs a funnel-shaped air injection manifold in conjunction with gas ports at the narrow end of the funnel and produces turndown ratios in the range of 5:1. Flame lengths and emissions of carbon monoxide and nitrogen oxides generally will satisfy the requirements of only the most basic industrial process.
As the air flows and fuel inputs increase, the ability of the simplest form of retention nozzle to retain the flame diminishes. This results in either a partial or total loss of flame at the nozzle. One type of a higher performance funnel-shaped retention nozzle uses air jets flowing radially in conjunction with a separate retention nozzle which acts to hold the flame inside the funnel section as fuel inputs and air flows are increased. The radial air jets do not intersect until they reach the centerline of the funnel and this tends to give a somewhat longer flame length and slower mixing of the air and fuel.
Another type of a high performance funnel-shaped retention nozzle incorporates a flame retention zone which has special baffling and porting to provide a stable flame in that area of the retention device. This type of nozzle may also use radial jets or it may incorporate tangential air jets which cause the air and fuel to spin and mix somewhat better than the radial jet type. Spinning induces better mixing and produces a somewhat shorter flame.
Both types of high performance retention nozzles mentioned above tend to produce a relatively large amount of fuel burning in the very center of the funnel-shaped combustor. The tangential air jet nozzle must have fairly thick walls on the funnel-shaped section in order to induce spin in the air jets. It is also limited in the angle of divergence of the funnel which makes a very deep funnel necessary if the diameter of the large end reaches any significant size. This is a drawback since larger diameters are necessary as burner inputs increase if flame length is to be kept short. In some industrial processes, it is necessary to make nozzles from heat-resistant stainless steel. With the necessary thick walls and depth of this type of nozzle, the cost of the unit may be too high to make it commercially competitive. The turndown range of this type of nozzle is approximately 40:1.
In order to ignite any burner, it is necessary to provide some sort of pilot which usually is ignited by an electric spark. This can either be a separate pilot or it can be a bypass pilot. Separate pilots are normally small premix type burners.
Bypass piloting is accomplished by admitting a small volume of gas into the main burner gas manifold and igniting it with an electric spark. This type of pilot is very popular because of lower costs and simpler piping. On multiple burner systems, the bypass pilot requires that some type of check valve be installed in the main gas line to restrict the traveling of the pilot gas from burner to burner. This adds to the cost and complexity of a burner system. Bypass pilot input tends to be somewhat higher than separate pilots, which means a higher input in the low firing position. This lowers the effective turndown ratio on the burner.